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    <script>

        function foo() {
            var a = 1 ;
            return function() {
                return ++a
            }
        }

        var res = foo() ;
        console.log(res());   // 2
        console.log(res());   // 3


        // 每一次调用函数都会形成新的执行空间   


        console.log(foo()());   // 重新调用了函数  a=1 


    </script>
    
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